The present invention relates to communication systems and more specifically to high power UHF filter elements.
Relays are used extensively for high power, UHF, switched-capacitor filters. The relay tuning time is presently restricted to approximately 1 millisecond. Switched-capacitor filters utilizing PIN-diodes can achieve tuning speeds of 100 microseconds or faster but require high-voltage and high-current power supplies to bias the PIN diodes in order to attain such high speed operation. Another limitation to contend with is that the current state of the art in PIN-diode technology does not support filter power-handling levels of 100 watts or greater. The highest power level for reasonable bandwidth, frequency-agile, switched-capacitor filters is approximately 25 watts, as typified by the F1556 which utilizes a bias supply of 500 Vdc and is available from the assignee of the present invention.
The F1556 filter represents a state of the art UHF switched-capacitor filter which uses PIN-diodes to achieve an approximate tuning time of 100 microseconds. Although the 100 microsecond tuning time can be met with PIN-diode switching, the resulting filter would require PIN-diode bias voltages greater than 1000 volts to achieve the 100 W power level. Such high switching voltage causes amplifier slew rate and EMI/noise problems. Moreover, suitable PIN-diodes with low "on" resistance, high "off" resistance, and 2000 V breakdown voltage are not presently available as commodity items.
Varactor tuned filters are an alternative to PIN-diode switched capacitor filters. Unfortunately, varactor diode tuned filters are currently limited to power levels less than 1 W because of the impracticality of building varactor devices with tuning voltages greater than 100 volts while retaining high unloaded Q (Qu). High-voltage varactors characteristically have high series resistance which degrades the Q.
Prior art in varactor diode tuning has included back-to-back varactors and some attempts at layering varactors to reduce the RF voltage across a given varactor. These prior attempts were typically unsuccessful, in part because standard miniature packaging concepts were not suitable for power applications that require special large scale packaging. In some instances, varactors were integrated into a size where the high lead inductance limited the tuning range as it approaches series resonance, and degraded the unloaded Q of the varactor because of the higher current and consequently conductor loss in the bond wires.
The solution to this large scale packaging problem is the novelty of the proposed invention.